Ecology is a fun unit that’s definitely easier than cell and molecular biology or animal anatomy and physiology, though you should still make sure you know the topics in Campbell’s quite well. For example, make sure you know the different biomes (and can name one based on a few points of description), the different geochemical cycles (and what processes cause the minerals to move from one form to another), and terminology describing the biosphere (like GPP, NPP, and NEP).
Some topics that aren’t well covered in Campbell’s, but can definitely show up on USABO semifinals exams are life tables, game theory, and statistics. One USABO semifinal exam dedicated a large portion of an FRQ section to life tables, so make sure to learn at least the basics. Game theory is more often tested at the USABO National Finals but you should know the basics just in case. The base topic for game theory is the prisoner’s dilemma, but game theory can also answer many questions in biological contexts. Finally, make sure you know basic statistics. Here are some base concepts you should definitely learn:
The difference between Type 1 and Type 2 error
The definition of variance and how it differs from standard deviation
When to use a T test vs a Chi Square test
Most of these are pretty straightforward, but make sure to learn them because some statistics usually shows up on USABO tests each year.